This invention relates to the field of electronic displays, and, more particularly, field emission display ("FED") devices.
As technology for producing small, portable electronic devices progresses, so does the need for electronic displays which are small, provide good resolution, and consume small amounts of power in order to provide extended battery operation. Past displays have been constructed based upon cathode ray tube ("CRT") or liquid crystal display ("LCD") technology. However, neither of these technologies is perfectly suited to the demands of current electronic devices.
CRT's have excellent display characteristics, such as, color, brightness, contrast and resolution. However, they are also large, bulky and consume power at rates which are incompatible with extended battery operation of current portable computers.
LCD displays consume relatively little power and are small in size. However, by comparison with CRT technology, they provide poor contrast, and only limited ranges of viewing angles are possible. Further, color versions of LCDs also tend to consume power at a rate which is incompatible with extended battery operation.
As a result of the above described deficiencies of CRT and LCD technology, efforts are underway to develop new types of electronic displays for the latest electronic devices. One technology currently being developed is known as "field emission display technology." The basic construction of a field emission display, or ("FED") is shown in FIG. 1. As seen in the figure, a field emission display comprises a face plate 100 with a transparent conductor 102 formed thereon. Phosphor dots 112 are then formed on the transparent conductor 102. The face plate 100 of the FED is separated from a baseplate 114 by a spacer 104. The spacers serve to prevent the baseplate from being pushed into contact with the faceplate by atmospheric pressure when the space between the baseplate and the faceplate is evacuated. A plurality of emitters 106 are formed on the baseplate. The emitters 106 are constructed by thin film processes common to the semi-conductor industry. Millions of emitters 106 are formed on the baseplate 114 to provide a spatially uniform source of electrons.
In order to cause the emitters to emit electrons, a plurality of electrodes are also formed on the baseplate. The electrodes are typically formed in a grid fashion with the row electrodes 108 formed on the baseplate and the column electrodes 110 formed on an insulator 116 attached to the baseplate.
FIG. 2 is a 3-dimensional cross-section showing the construction of row electrodes 202 and column electrodes 204. When a differential voltage is applied between a row electrode and a column electrode, an electric field is created at the tip of the emitters located at the intersection of the row and the column. The electric field at the tip of the emitter is controlled by the sum of the row and column voltages and is sufficiently high to cause electrons to tunnel through the surface of the emitter, into the vacuum, with no loss of energy. Virtually all the electrons bombard the phosphor, resulting in a bright display. Gray-scale or color can be achieved by varying the voltage applied to the column.
The number of row and column electrodes required will depend on the number of individual display elements, or "pixels," to be addressed by the electrodes. FIG. 3 illustrates the row and column electrodes required for a standard VGA display having 640 columns by 480 rows. Additionally, for a color display, each column requires a separate electrode for red, green, and blue elements. Therefore, a total of 1920 column electrodes are required.
A drive circuit is required to generate the desired voltage differential between each of the row and column electrodes. In a "passive matrix" drive scheme, each conductor requires a separate drive circuit. Referring still to FIG. 3, an image is created on an FED by sequentially "scanning" the rows. First, a voltage source 300 is used to apply a voltage row 302-1 to drive it to the appropriate voltage level. Second, all columns 304-1 to 304-1920 are driven to a voltage level related to the desired brightness of the relevant pixel using a circuit known as a "pulse height modulator" (not shown). The modulator sends pulses to its corresponding column electrode (304-1 to 304-1920) in which the height of the pulses depends on the desired brightness of the pixel. Third, all columns are turned off and row 302-1 is turned off. Finally, row 302-2 is then turned on and the process is repeated for rows 302-2 through 302-480. FIG. 3A is a timing diagram showing the column pulse height in conjunction with example voltages at rows 1 and 2.
However, this method of supplying a differential voltage to the electrodes is inefficient because each time a new row is scanned the columns must be discharged and then recharged to the desired voltages by the pulse height modulator. In fact, it is possible to calculate how much energy is required using this method.
For example, the above sequence occurs sixty times a second. So row 302-1 will also turn on and off sixty times in one second. A standard VGA display contains 640 columns by 480 rows. Therefore, the maximum pulse width of each row is 1/60(480)=34.7 microseconds.
Referring again to FIG. 3, a capacitance 306 will be associated with each intersection of a row and column. Therefore, in column 1, the total capacitance is the parallel combination of C1R1+C1R2+C1R3+ . . . +C1R480, where CxRy is the capacitance at column x, row y. This total capacitance can equal as much as 1 nanofarad, possibly more, depending on the area of the display.
The amount of current required to drive each column is represented by the relationship: EQU .DELTA.V/.DELTA.T=I/C.
Example values for theses parameters would be: EQU .DELTA.V=50 volts, EQU .DELTA.T=5 microseconds, and EQU C=1 nanofarad.
Therefore, solving the equation for I yields: I=10 milliamps.
Accordingly, the power required to drive 1 column is calculated as follows: EQU P=IV Duty Cycle=10 milliamps.multidot.50 volts.multidot.(5/34.7)microseconds=71 milliwatts.
Thus, the total power requirement for the FED would be: EQU P.sub.total =P.multidot.number of columns=71 milliwatts.multidot.1920=137 watts.
This type of power requirement represents a heavy drain on the batteries and renders them useless for such an application.
Attempting to overcome the above-mentioned problems by replacing the pulse width modulators with analog amplifier circuits has heretofore been impractical because continuously operating the amplifiers at the required current levels wastes large amounts of current in the devices which comprise the amplifier. Also, power amplifiers are packaged independently, whereas existing display drivers have multiple outputs per chip.
Therefore, it is an object of the present invention to overcome the above shortcomings.